1. Field
The present disclosure relates generally to the field of anti-reflective, anti-glare, barrier coatings applied to glass, metals and plastics in surfaces, windows, windshields, screens, displays, architecture, goggles, eyeglasses, etc., in particular to glass used as the front cover of solar modules, high-transparency glass used for display purposes such as protective covers for works of art, museum display glass, commercial display glass, front glass of electronic screens or instrument panels that need anti-glare properties and automotive glass. Specifically, it relates to soluble high silanol containing silsesquioxane compositions and methods for their preparation for use as thin-film coatings.
2. Description of Related Art
The interface between air and the surface of typical glass such as soda-lime glass will reflect about 4% of normally incident light. This reflection is caused by the difference in refractive index of the glass, approximately 1.52 and the air approximately 1.0. In order reduce the reflection, the glass may be coated with an anti-reflective coating that has an intermediate refractive index between about 1.25 and about 1.45.
Several methods exist to create single-layer anti-reflection coatings using sol-gel chemistry. Commonly used methods can be broadly sorted into five groups. The first group: using solid silica nanoparticles have been known for a long time, for example U.S. Pat. No. 2,432,484 filed in 1943 teaches using a solution containing colloidal silica nanoparticles to create an anti-reflective coating on glass. Another more recent example is U.S. Pat. No. 7,128,944 which teaches a porous SiO2 layer created by depositing an aqueous solution containing silica particles that is then sintered at temperatures of at least 600° C. The second group teaches using porogenic materials to increase the porosity of sol-gel derived thin-films. As porosity increases, refractive index is reduced. In general, porogenic materials are high molecular weight organic compounds that create networks of small pores when they are removed by thermal processing at high temperatures. For example, European patent application no. EP1329433 teaches using Polyethyleneglycol tert-octyl phenyl ether (Triton) in high concentrations as a porogen with subsequent thermal sintering that causes combustion of the porogen to increase porosity. A major drawback of this approach is that the small pores created adsorb water, which can greatly diminish the performance of the anti-reflective coating. It has been recognized that one method of mitigating this problem is to create coatings with relatively large pores. While the inner surface of the pore may adsorb water, the main void remains empty. The third group teaches creating large pores using solid or liquid templates around which the film is formed after which high temperature processing burns away the template or a solvent dissolves away the template leaving a void. For example, U.S. patent application Ser. No. 12/514,361 teaches using particulate (quasi)spherical nano-particles composed of polymethyl methacrylate (PMMA) or nano-droplets of an oil as a pore forming agent that templates a large pore and that are removed either by washing with a solvent such as THF at low temperature or are preferably burned away at high temperature. The fourth group is a variant of the third, in that a solid polymer nano-particle is used to template a void. However, in this case, sometimes referred to as core/shell, the nano-particle is coated in a shell of, for example, silica before being embedded in a matrix material. High temperature is again used to burn out the nano-particle polymer core or template to leave a void behind. For example, U.S. patent application Ser. No. 12/438,596 teaches coating a substrate using hollow particles in a binder and then curing to create an anti-reflective coating. The last group is again a variant of the template method, however in this case hollow silica particles are formed that already have an internal void, that are then embedded in a matrix to form a porous coating. For example, PCT patent application no. PCT/JP2013/001114 teaches a method of producing hollow particles with little or no aggregation and then producing an anti-reflection coating using those particles.
Many commercially available anti-reflective coating materials for the glass and photovoltaic solar module industries utilize these or similar methods. Notwithstanding that some art teaches using a solvent to remove pore-forming agents at low temperatures, this is not generally a practical method at industrial scale as it is slow and requires large quantities of organic solvents. It is therefore common in industrial scale applications of these coating technologies to use a high temperature sintering or curing step at between 400° C. and 750° C. As this processing step is also commonly used as a tempering step to mechanically strengthen the glass, both processes are accomplished at the same time.
Anti-reflective coatings that are cured during the glass tempering process or by other similar high temperature processing share a number of common features. First, they are hydrophilic or super-hydrophilic with water contact angles as measured by a goniometer of less than 60° and less than 20° respectively. The high temperature oxidizes all organic components of the coating, leaving behind almost pure silica. Second, they are frequently quite brittle. Their mechanical strength is derived to some extent by sintering of the coating, too much sintering reduces the coating's porosity and, hence, optical performance. Therefore, a balance must be achieved between optical and mechanical performance that frequently leaves the mechanical performance at less than ideal. In general, these brittle hydrophilic coatings are prone to degradation caused by soiling and abrasion when subject to long-term exposure to many outdoor environments.
It would be desirable to make coatings with large pores and high optical performance and abrasion resistance at a practical, low temperature process at industrial scale that does not require solvent removal and yet still imparts good mechanical properties while preserving organic elements and properties in the coating such as hydrophobicity or oleophobicity.